Gas generators for safety devices usually contain a solid propellant on the basis of sodium azide as a gas-yielding main constituent. However, sodium azide is toxic and can easily react with heavy metals to form extremely dangerous, violently reacting compounds. Therefore, special measures are necessary both during the preparation of the gas-producing mixtures and during the disposal of defective or unused gas generators.
The combustion of sodium-azide-containing propellants also produces a number of solid substances which have to be removed from the gas stream by means of suitable filtering devices in the gas generator, or have to be retained in the gas generator. Therefore, attempts have already been made to produce propellants which burn off in a substantially residue-free manner.
U.S. Pat. No. 5,545,272 describes a gas-producing composition substantially consisting of 35 to 55 wt. % nitroguanidine and approximately 45 to 65 wt. % phase-stabilised ammonium nitrate. The addition of phase-stabilising agents to the ammonium nitrate is considered necessary because a structural change, occurring in pure ammonium nitrate at 32.3.degree. C., is linked with an increase in volume which, according to the aforementioned patent specification, is to be prevented. Potassium salts such as potassium nitrate and potassium perchlorate, but also potassium dichromate, potassium oxalate and mixtures thereof in a quantity of 5 to approximately 25 wt. %, preferably 10 to 15%, are named as phase-stabilising additives.
However, the gas-producing composition of U.S. Pat. No. 5,545,272 cannot burn off residue-free, i.e. without solid combustion residues, on account of the relatively high proportion of phase-stabilising additives for the ammonium nitrate. Consequently, the combustion of this propellant gives rise to smoke formation which can only be partially suppressed by filtering devices in the gas generator. Furthermore, the known mixture is only poorly ignitable and has a relatively low rate of combustion. Moreover, the high proportion of ammonium nitrate leads to high hygroscopicity, necessitating a high level of expenditure for processing, storage and use in the gas generator in order to prevent unwanted changes occurring in the propellant mixture.
DE 195 05 568 A1 discloses a propellant for gas generators, comprising, as fuel, at least one compound from the group of tetrazoles, triazoles, triazines, cyanic acid, urea, derivatives thereof, derivatives or salts thereof, and, as oxidants, at least three compounds from the group of peroxides, nitrates, chlorates or perchlorates, as well as combustion moderators and additions reducing the proportion of toxic gases. Biuret, guanidine, nitroguanidine, guanidine nitrate, aminoguanidine, aminoguanidine nitrate, thiourea and triaminoguanidine nitrate can be used as urea derivatives. Ammonium nitrate or ammonium perchlorate, among others, are named independently of one another as oxidants. However, DE 195 05 568 A1 contains no indication of the proportion of solid substances in the gas mixture released after the combustion of the propellants.
Therefore, there is still a need for gas-generator propellants which burn off in a residue-free manner, are non-toxic, i.e. are free of azides and heavy metals, burn off with a hundred percent gas yield to form a particle-free, non-toxic combustion gas, and have a sufficiently high rate of combustion and good thermal and chemical stability.